Mössbauer spectroscopy analysis of iron compounds in carboxylated multiwall carbon nanotubes and their ammonium salt

Abstract

Different forms of iron compounds embedded in non‐functionalized (as prepared) and functionalized multiwall carbon nanotubes (MWCNTs) were studied. Iron used to catalyze MWCNT synthesis can be found inside nanotubes and built‐in within their walls. Mössbauer spectroscopy was applied to study valence and spin states as well as magnetic ordering of the iron compounds. In the Mössbauer spectra of as prepared MWCNTs the Fe3C carbide provides the main contribution. The content of Fe3C decreases by about 20 and 40% in carboxylated carbon nanotubes (MWCNTs‐COOH) and ammonium salt of carboxylated carbon nanotubes (MWCNTs‐COONH4), respectively. A small amount of α‐Fe and FexCx iron forms are always observed in all studied carbon nanotubes. In MWCNTs‐COOH, additionally, ferrihydrates and/or FexCy with oxygen in the second coordination sphere of iron are present (at a level of about 17%). In MWCNTs‐COONH4 their content increases by a factor of 3. The experimental data shows that the purification and functionalization of as prepared MWCNTs result in removal of about 90% of iron contaminations. Additionally, we observe the modification of iron compounds inside MWCNTs. All these results are important in optimizing the as prepared MWCNT purification process and in finding a new way to produce different Fe phases inside MWCNTs.

Influence of iron contaminations on local and bulk magnetic properties of nonfunctionalized and functionalized multi‐wall carbon nanotubes

“…Most probably they are related to the Fe clusters, which contamination in the nonfunctionalized MWCNTs is about 10 times higher than in the chemically treated MWCNTs. Their distribution is comparable to that of the small structures observed in the TEM pictures .…”

Studies of Fe‐binding sites within multiwall carbon nanotubes using Mössbauer spectroscopy

“…Moreover, a comparison of these data with the results described in Ref. 53 shows that temperature is also a very crucial parameter in the process of purification. When HNO 3 was used as an oxidant but at a temperature lower by about 20 °C fractions of γ‐Fe and Fe 2 O 3 particles were not detected at all 53.…”

Preparation and Characterization of Hematite-Multiwall Carbon Nanotubes Nanocomposite

“…So far, it has been reported that the purification of contaminated CNTs could be achieved via the functionalization of carbon nanotubes [9]. Through this procedure, it is possible to cover the CNTs surface by various chemical groups such as: -OH, -COOH, COONH 4 , etc.…”